Control of lo signal frequency offset between optical transmitters and receivers

ABSTRACT

A system includes a laser configured to generate a tunable optical frequency. The system also includes an optical transmitter to map baseband data to symbols represented in a digital modulation constellation, add a frequency offset to the digital modulation constellation to cause the digital modulation constellation to rotate at a rate equal to the added frequency offset, modulate the optical frequency with the rotating digital modulation constellation, and transmit the resulting modulated optical frequency. The system also includes an optical receiver to receive the transmitted modulated optical frequency and, using the tunable optical frequency, detect the rotating digital modulation constellation conveyed by the received modulated optical frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/921,222, filed Oct. 23, 2015, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to optical communications.

BACKGROUND

An optical transmitter may employ digital modulation to modulatetransmitter laser light (i.e., an optical signal), to produce modulatedlaser light. In an example, digital modulation in the form ofdual-polarization binary phase shift keying (DP-BPSK) modulates thelaser light only on one axis, compared with other types of digitalmodulation, such as quadrature phase shift keying (QPSK) and 16quadrature amplitude modulation (16QAM), which modulate the transmitterlaser light onto orthogonal axes. An optical receiver demodulates themodulated laser light based on receiver laser light. Ideally, there is anon-zero frequency offset between the transmitter laser light and thereceiver laser light, which improves demodulation. In practice, however,there may be zero or near-zero frequency offsets between the transmitterand receiver laser light. This occurs when the optical transmitter andreceiver use different lasers that happen to generate laser light at thesame frequency by chance or when the optical transmitter and receivershare a common laser.

To demodulate the modulated optical laser light, the optical receiverdetects in-phase (I) and quadrature (Q) signals conveyed by themodulated optical laser light based on the receiver laser light. Theabove mentioned zero or near-zero frequency offsets cause energy fadingin the I and Q signals, which hampers the demodulation process. Forexample, the fading perturbs automatic gain control loops used tocontrol photo-detectors that detect the I and Q signals. The fading alsocomplicates polarization tracking in cases where a constant modulusequalization algorithm is used. The fading is exacerbated by DP-BPSKmodulation, which already limits energy to only one axis (either I orQ).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical communication system, accordingto an example embodiment.

FIG. 2A is a digital modulation constellation for binary phase shiftkeying (BPSK) produced by an optical transmitter in the system of FIG.1, according to an example embodiment.

FIG. 2B is a digital modulation constellation for quadrature phase shiftkeying (QPSK) produced by the optical transmitter, according to anexample embodiment.

FIG. 2C is a digital modulation constellation for quadrature amplitudemodulation (QAM) produced by the optical transmitter, according to anexample embodiment.

FIG. 3 is a detailed block diagram of the optical transmitter and anoptical receiver in the communication system, according to an exampleembodiment.

FIG. 4A is a detailed block diagram of a transmitter digital processorof the optical transmitter, according to an embodiment.

FIG. 4B is a diagram of frequency spectrum plots of a modulated opticalsignal and a frequency offset modulated optical signal produced by theoptical transmitter, according to an example embodiment.

FIG. 5 is a detailed block diagram of a receiver digital processor ofthe optical receiver, according to an embodiment.

FIG. 6 is a frequency plot of target windows for local oscillator(LO)-signal frequency offset used in methods of controlling LO-signalfrequency offset, according to an embodiment.

FIG. 7 is a flowchart of a method of controlling an LO-signal frequencyoffset between a transmit optical frequency used to generate a modulatedoptical frequency (that is transmitted) and a receive LO frequency usingthe target windows of FIG. 6 that is performed in the optical receiverof FIG. 3, according to an embodiment.

FIG. 8 is a flowchart of a transmit method of generating a modulatedoptical signal with an LO-signal frequency offset performed in theoptical transmitter, according to an embodiment.

FIG. 9 is a flowchart of a receive method of controlling an LO-signalfrequency offset performed in the optical receiver, according to anembodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

A system includes a laser configured to generate a tunable opticalfrequency. The system also includes an optical transmitter to mapbaseband data to symbols represented in a digital modulationconstellation, add a frequency offset to the digital modulationconstellation to cause the digital modulation constellation to rotate ata rate equal to the added frequency offset, modulate the opticalfrequency with the rotating digital modulation constellation, andtransmit the resulting modulated optical frequency. The system alsoincludes an optical receiver to receive the transmitted modulatedoptical frequency and, using the tunable optical frequency, detect therotating digital modulation constellation conveyed by the receivedmodulated optical frequency.

Example Embodiments

Referring to FIG. 1, there is shown a block diagram of an exampleoptical communication system 100 including a first opticaltransmitter/receiver (“transceiver”) 102 and a second opticaltransceiver 104 that exchange optical signals with each other over anoptical medium such as free-space and/or an optical fiber (notspecifically shown in FIG. 1). Optical transceiver 102 includes anoptical transmitter (TX) TX1, an optical receiver (RX) RX1, and afrequency tunable laser L1 to provide a laser signal LO1 (also referredto as an “optical signal LO1”) to optical transmitter TX1 and opticalreceiver RX1. In the example of FIG. 1, optical signal LO1 is used bothas an optical carrier for modulation in optical transmitter TX1 and as alocal oscillator (LO) signal in optical receiver RX1; however, inanother example, separate laser signals may be used in the opticaltransmitter and the optical receiver. Similarly, optical transceiver 104includes an optical transmitter TX2, an optical receiver RX2, and afrequency tunable laser L2 to provide a laser signal LO2 (also referredto as “an optical signal LO2”) to optical transmitter TX2 and opticalreceiver RX2. In the example of FIG. 1, optical signal LO 2 is used bothas an optical carrier for modulation in optical transmitter TX2 and asan LO signal in optical receiver RX2; however, in another example,separate laser signals may be used in the optical transmitter and theoptical receiver. In an example, optical signals LO1 and LO2 haverespective first and second (center) optical frequencies near 193.1THz+/−1.5 GHz, although other optical frequencies are possible.

Moving left-to-right in FIG. 1, optical transmitter TX1 receivestransmit data 110 having a frequency spectrum centered at 0 Hz, i.e.,baseband. Optical transmitter TX1 modulates laser signal LO1 withbaseband transmit data 110 to produce a modulated optical signal 112.Optical transmitter TX1 transmits modulated optical signal 112 over theoptical medium. In an example, optical transmitter TX1 modulates lasersignal LO1 with transmit data 110 using any known or hereafter developedtype of digital modulation, including binary phase shift keying (BPSK),dual-polarization (DP) BPSK (DP-BPSK), quadrature phase shift keying(QPSK), quadrature-amplitude modulation (QAM), or the like.

Optical receiver RX2 receives modulated optical signal 112, and usesoptical signal LO2 as a local oscillator signal to demodulate themodulated optical signal so as to recover receive data 120 therefrom,where the receive data is representative of transmit data 110. Ideally,the first and second optical frequencies of optical signals LO1 and LO2are different, i.e., there is a non-zero LO-signal frequency offsetbetween the first and second optical frequencies, so that opticaltransmitter TX1 and optical receiver RX2 perform their respectivemodulation and demodulation operations based on the different first andsecond optical frequencies. Such an LO-signal frequency offset (alsoreferred to simply as a “frequency offset”) advantageously spreadsaverage energy of modulated optical signal 112 across multiplephoto-detectors in optical receiver RX2 that detect the energy used todemodulate the (received) modulated optical signal. If left to chance,the first and second frequencies may be the same, i.e., there may be noLO-signal frequency offset between the first and second frequencies,which complicates demodulation in optical receiver RX2.

According to a first embodiment presented herein (also referred to as a“receiver implemented” embodiment), optical receiver RX2 tunes thefrequency of optical signal LO2 relative to the frequency of opticalsignal LO1 while the optical receiver demodulates (received) modulatedoptical signal 112, so as to impose a non-zero LO-signal frequencyoffset (e.g., an LO-signal frequency offset near 800 MHz) between thefrequencies of optical signals LO1 and LO2. To do this, optical receiverRX2 derives an LO frequency control signal TC2 that tunes the frequencyof optical signal LO2, so as to drive the LO-signal frequency offsetbetween the frequencies of optical signals LO1 (which is associated withthe optical transmitter TX1) and LO2 (which is associated with theoptical receiver RX2) toward one of multiple non-overlapping, non-zerotarget windows of LO-signal frequency offset. Control signal TC2 maydrive an operating bias current supplied to laser L2 to tune itsfrequency, or drive a current that controls a temperature of laser L2 totune its frequency.

Optical transmitter TX2 and RX1 are configured and operate similarly tooptical transmitter TX1 and optical receiver RX2, respectively. Thusoptical transmitter TX2 modulates optical signal LO2 with basebandtransmit data 130 to produce a modulated optical signal 140, andtransmits the modulated optical signal over the optical medium. Opticalreceiver RX1 receives modulated optical signal 140, and demodulates itusing optical signal LO1 as a local oscillator signal to recover receivedata 142 representative of transmit data 130. While optical receiver RX1processes (received) modulated optical signal 140, the optical receivermay tune the frequency of optical signal LO1 relative to the frequencyof optical signal LO2 (associated with optical transmitter TX2) using anLO frequency control signal TC1 to ensure a sufficient non-zeroLO-signal frequency offset between the frequencies of optical signalsLO1 and LO2.

In the example of FIG. 1, laser L1 provides signal LO1 to both opticaltransmitter TX1 and optical receiver RX1, and laser L2 provides signalLO2 to both optical transmitter TX2 and RX2. In an alternative examplementioned above, different lasers may drive the signals to opticaltransmitter TX1 and optical receiver RX1, and different lasers may drivethe signals to optical transmitter TX2 and optical receiver RX2. Theexample of FIG. 1 and the alternative example may each employ theabove-described receiver implemented embodiment to tune the frequency ofthe optical signal (e.g., LO1 or LO2) at the optical receiver so as tointroduce a non-zero LO-signal frequency offset between the opticalsignal at the optical receiver data and the received modulated signalfrom the optical transmitter.

Optical transceivers 102 and 104 may also each operate in a loop-backarrangement. In the loop-back arrangement, an optical output of anoptical transmitter of a given optical transceiver is supplied to anoptical input of that optical transceiver. Such a configuration isdepicted at dotted-line 150 in FIG. 1. Dotted-line 150 represents aloop-back connection from an optical output of optical transmitter TX1to an optical input of optical receiver RX1, such that opticaltransmitter TX1 transmits modulated optical signal 112 and opticalreceiver RX1 receives that same signal. The loop-back arrangement may beuseful for testing a given optical transceiver, e.g., opticaltransceiver 102. In the loop-back arrangement of FIG. 1, opticaltransmitter TX1 and optical receiver RX1 share optical signal LO1. Thus,optical transmitter TX1 performs modulation operations and opticalreceiver RX1 performs demodulation operations using the same opticalsignal, i.e., optical signal LO1. That is, the laser (i.e., optical)frequency used to modulate in optical transmitter TX1 is the same asthat of the local oscillator used to demodulate in optical receiver RX1,which can complicate demodulation in the optical receiver.

In the loop-back arrangement, tuning the frequency of the optical signal(e.g., LO1 or LO2) at the optical receiver as described above does notachieve the desired LO-signal frequency offset because the opticaltransmitter and the optical receiver use the same optical signal.Accordingly, a second embodiment associated with the loop-backarrangement (also referred to as the “loop-back” embodiment) is used toensure that a sufficient LO-signal frequency offset exists in loop-back.Briefly, the optical transmitter (e.g., optical transmitter TX1)incorporates a small, digitally implemented frequency offset (e.g.,approximately 7 or 8 MHz) into the modulated optical signal (e.g.,modulated optical signal 112), which is detected in the looped-backoptical receiver (e.g., optical receiver RX1), as will be described morefully below. The small frequency offset need only be larger than acontrol bandwidth of gain control loops of the optical receiver, whichare typically on the order of 500 KHz. In the loop-back embodiment, theoptical transmitter (e.g., TX1 or TX2) may add the small frequencyoffset only when the optical transmitter is configured for loop-backwith the corresponding optical receiver (e.g., RX1 or RX2).Alternatively, the optical transmitter (e.g., TX1 or TX2) may always addthe small frequency offset, even when not in loop-back, so that there isno need for a special configuration just for loop-back.

As mentioned above, optical transmitters TX1 and TX2 may modulaterespective optical signals LO1 and LO2 using digital modulation, such asBPSK, DP-BPSK, QPSK, QAM, or the like. With reference now to FIGS.2A-2C, there are shown digital modulation constellations or In-phase(I)-Quadrature (Q) symbol plots for various types of digitalmodulations, where the vertical and horizontal axes correspond to I andQ axes, respectively. With reference to FIG. 2A, there is shown anexample digital modulation constellation 200 for BPSK and DP-BPSK wherethe second polarization state has an identical constellation. Withreference to FIG. 2B, there is shown an example digital modulationconstellation 210 for QPSK. With reference to FIG. 2C, there is shown anexample digital modulation constellation 220 for 16 QAM.

Optical transceiver 102 is now described in detail. Because opticaltransceivers 102 and 104 are configured and operate similarly to eachother, the ensuing description of optical transceiver 102 and itsvarious components, and the methods implemented therein, shall sufficeas a description for optical transceiver 104.

With reference to FIG. 3, there is an example detailed block diagram ofoptical transmitter TX1 and optical receiver RX1 of optical transceiver102. Optical transmitter TX1 includes (i) an electrical/optical (E/O)converter 302 to convert electrical signals to optical signals, and (ii)a transmitter digital processor 304. Optical receiver RX1 includes (i)an optical/electrical (O/E) converter 306 to convert optical signals toelectrical signals, and (ii) a receiver digital processor 308. Digitalprocessors 304 and 308 operate in a digital domain to process digitaland/or digitized signals at or near baseband.

Referring first to optical transmitter TX1, transmitter digitalprocessor 304 modulates transmit data 110 to produce a set of I and Qmodulation signals or channels XI, XQ, YI, and YQ (indicatedcollectively at TIQ) representative of a digital modulationconstellation, such as any of digital modulation constellations 200,210, and 220, for example. The “X” and “Y” labels denote differentpolarization states, e.g., orthogonal polarization states. E/O converter302 includes amplifiers 310 and an optical modulator 312. Amplifiers 310amplify respective ones of I and Q modulation signals XI, XQ, YI, and YQand provide respective amplified versions of the I and Q modulationsignals to optical modulator 312. Optical modulator 312 directs opticalsignal LO1 through parallel optical transmit paths TP1-TP4 eachincluding a respective one of optical modulator devices M1-M4 (e.g.,Mach-Zehnder optical modulator devices) that modulates optical signalLO1 in that path responsive to a corresponding one of the respectiveamplified versions of I and Q modulation signals XI, XQ, YI, and YQ, tothereby produce a respective one of multiple modulated optical signalcomponents. Parallel optical transmit paths TP1-TP4 feed the multiplemodulated optical signal components to a polarization beam combiner 314that combines the components into modulated optical signal 112, which isthen transmitted. In an example where optical modulator devices M1-M4include Mach-Zehnder optical devices, each Mach-Zehnder optical deviceis incorporated into a bias control loop having a response bandwidth onthe order of several hundred KHz.

Optical receiver RX1 is now described. Assuming optical transceiver 102is not configured for loop-back, O/E converter 306 of optical receiverRX1 receives modulated optical signal 140 from optical transmitter TX2,for example. O/E converter 306 includes a polarization beam splitter(PBS) 320 to direct (received) modulated optical signal 140 to parallel90° hybrid splitters H1 and H2 each of which also receives optical localoscillator (LO) signal LO1. 90° hybrid splitters H1 and H2 areconfigured as an optical mixer or frequency down-converter tofrequency-mix modulated optical signal 140 with optical signal LO1 andthereby frequency down-convert the modulated optical signal towardbaseband, to produce sets of frequency down-converted mixer signalproducts MSP1 and MSP2. In an example in which optical signal LO1 (inRX1) has a frequency F_(rx1) and optical signal LO2 (in TX2) has afrequency F_(tx2), the frequency down-converted mixer signal productsinclude a frequency difference or offset F_(tx2rx1)=F_(tx2)−F_(rx1).

O/E converter 306 also includes sets of photo-detectors (e.g., PIN/TIAoptical detectors) PD1 and PD2 to detect respective ones of the sets ofmixer signal products MSP1 and MSP2, to produce a set of detected I andQ signals X0, X90, Y0, and Y90 (collectively indicated at RIQ)representing a detected optical field, and provides the detected I and Qsignals X0, X90, Y0, and Y90 to receiver digital processor 308.Photo-detectors PD1 and PD2 are each associated with a control loopresponse bandwidth of hundreds of KHz. Receiver digital processor 308recovers receive data 142 from detected I and Q signals X0, X90, Y0, andY90 and derives control signal TC1 to tune the frequency of opticalsignal LO1 to achieve the desired LO-signal frequency offset between thefrequencies of optical signals LO1 and LO2 (where LO2 was used togenerate modulated optical signal 140 in optical transmitter TX2) asdiscussed briefly above and in further detail below.

With reference to FIG. 4A, there is an example detailed block diagram oftransmitter digital processor 304. Techniques implemented in transmitterdigital processor 304 may be used in combination with the receiverimplemented embodiment, and may also be used in the loop-back embodimentbriefly described above in connection with FIG. 1. Transmitter digitalprocessor 304 includes an encoder 406, a symbol mapper 408, a frequencyrotator 410, a pulse shaper 412, multiple digital-to-analog converters(DACs) 414 (e.g., 4 DACs), and a processor 416 to control theaforementioned components. Encoder 406 encodes baseband transmit data110 to produce encoded data. Symbol mapper 408 maps the encoded data todigital modulation symbols that represent a baseband/stationary digitalmodulation constellation. As part of the loop-back embodiment, frequencyrotator 410 adds a small digitally implemented frequency offset orfrequency shift to the digital modulation constellation based on anfrequency offset control signal from processor 416, which causes theconstellation to rotate at a rate equal to the added frequency offset.Pulse shaper 412 pulse shapes the symbols of the rotating digitalmodulation constellation. DACs 414 convert the pulse-shaped symbols fromthe digital domain to the analog domain to produce the set of I and Qmodulation signals XI, XQ, YI, and YQ as analog (or continuous time)signals representative of the rotating digital modulation constellation,and provide the digitized I and Q modulation signals to E/O converter302.

With reference to FIG. 4B, there are shown example frequency spectrumplots of modulated optical signal 112. Frequency spectrum 450 representsmodulated optical signal 112 having a modulated signal spectrum centeredat the frequency LO1, i.e., without any frequency offset added thereto.On the other hand, frequency spectrum 460 represents modulated opticalsignal 112 with a frequency offset added thereto via frequency rotator410, as described above. Note that frequency spectrum plot 460 of FIG.4B shows an exaggerated LO-signal frequency offset of 1 GHz (1000 MHz)for illustration purposes only, because smaller frequency offsets wouldnot be sufficiently large to show separation between spectrums 450 and460 given the x-axis frequency scale of the spectrum plots. In practice,a relatively small frequency offset of approximately 7-8 MHz is usedsuch that spectrums 450 and 460 would be separated only by that amount.

The small frequency offset imposed by frequency rotator 410 is greaterthan (i) the bias control loop bandwidth of several hundred KHzassociated with the optical modulator devices M1-M4 and therefore doesnot interfere with the optical modulation process, and (ii) the controlloop response bandwidth of hundreds of KHz associated withphoto-detectors PD1 and PD2 and therefore does not interfere with theoptical detection processes in the photo-detectors.

With reference to FIG. 5, there is an example detailed block diagram ofreceiver digital processor 308. Receiver digital processor 308 recoversreceive data 142 from I and Q signals X0, X90, Y0, and Y90 provided byO/E converter 306. Receiver digital processor 308 also derives LOfrequency control/tune signal TC1, based on I and Q signals X0, X90, Y0,and Y90, to tune the frequency of optical signal LO1 so as to drive theLO-signal frequency offset between the frequencies of optical signalsLO1 and LO2 to one of the target windows, as mentioned briefly above inconnection with FIG. 1 and further below. Note that in loop-back, inwhich optical receiver RX1 receives modulated optical signal 112 fromoptical transmitter TX1 instead of modulated optical signal 140 fromoptical transmitter TX2, there is no LO-signal frequency offset betweenthe optical transmitter optical signal used as a carrier (in the opticaltransmitter) and the optical receiver optical signal used as an LOsignal (in the optical receiver); instead there may be only the smallLO-signal frequency offset imposed by frequency rotator 410 in opticaltransmitter TX1 as part of the loop-back embodiment.

Receiver digital processor 308 includes analog-to-digital converters(ADCs) 502 (e.g., 4 ADCs), a chromatic dispersion compensation filter(“CD comp”) 504, a frequency compensator (Freq comp”) 506, apolarization compensator (“Pol comp”) 508, a carrier recovery module510, a forward error corrector 512, a processor 514 to control theaforementioned components and derive LO frequency control signal TC1,and a memory 518. Memory 518 may comprise one or more tangible computerreadable storage media encoded with software comprising computerexecutable instructions and when the software is executed (by processor514) it is operable to perform operations described herein. Memory 518stores control logic 520 to perform methods/operations described herein,including methods to control LO-signal frequency offset described belowin connection with FIGS. 7 and 9. Memory 518 also stores data 522 usedand generated by logic 520, such as information defining target windowsfor LO-signal frequency offsets.

ADCs 502 digitize respective ones of I and Q signals X0, X90, Y0, andY90, and provide the digitized signals to CD comp 504. CD comp 504processes the digitized I and Q signals to compensate for pulse spreadintroduced into modulated optical signal 140 in the optical medium(e.g., optical fiber) due to chromatic dispersion. CD comp 504 mayinclude a Finite Impulse Response (FIR) filter implemented in thefrequency domain. CD comp 504 provides pulse compensated signals tofrequency compensator 506. Frequency compensator 506, polarizationcompensator 508, and carrier recovery module 510 together implement acarrier recovery loop to determine/measure the LO-signal frequencyoffset between the frequencies of optical signals LO1 and LO2 (and anyadditional small LO-signal frequency offset introduce by frequencyrotator 410) as represented in I and Q signals X0, X90, Y0, and Y90, andreport the determined LO-signal frequency offset (indicated at 520) toprocessor 514.

More specifically, frequency compensator 506 removes most of theLO-signal frequency offset (e.g., so that only a few MHz residual offsetremain) and any additional small frequency offset introduced byfrequency rotator 410 based in part on feedback from carrier recoverymodule 510. Carrier recovery module 510 compensates for/removes anyresidual LO-signal frequency offset and phase offset remaining afterfrequency compensator 506, and provides a report of the residualfrequency and phase offsets to frequency compensator 506 as thefeedback. Frequency compensator 506 reports the LO-signal frequencyoffset (indicated at 520) to processor 514. Polarization compensator 508(between frequency compensator 506 and carrier recovery module 510)corrects the polarization state of the received modulated opticalsignal, so that its X and Y polarization components are correctlyseparated even though the received modulated optical signal polarizationstate may be arbitrary. Polarization compensator 508 may include abutterfly structure of FIR filters which may be implemented in the timeor frequency domain.

The carrier recovery loop (comprising modules 506, 508, and 510) removesthe remaining LO-signal frequency offset, and any phase error, toproduce baseband data, and provides the baseband data from carrierrecovery module 510 to error corrector 512. Error corrector 512 performserror correction on the baseband data, to produce receive data 142.

According to the receiver implemented embodiment, processor 514 tunesthe frequency of optical signal LO1 to ensure a non-zero LO-signalfrequency offset exists between the frequencies of optical signals LO1and LO2 (as manifested at photo-detectors PD1 and PD2). At a high-level,processor 514 defines predetermined non-overlapping target or desiredwindows that cover desired non-zero ranges of LO-signal frequency offsetbetween the frequencies of optical signals LO1 and LO2. Processor 514receives LO-signal frequency offset measurement 520. If the LO-signalfrequency offset indicated in the measurement 520 is not within one ofthe target windows, processor 514 derives LO frequency control signalTC1 so as to tune the frequency of optical signal LO1 in a directionthat drives the LO-signal frequency offset toward a nearest one of thetarget windows.

With reference to FIG. 6, there is a frequency plot 600 that showsexample target windows of LO-signal frequency offset 605 and 610.“Positive” target window 605 (also referred to as “window 1”) has afrequency width h (i.e., covers a range of frequencies h) that iscentered at a target LO-signal frequency offset of +F_(targ) frombaseband. “Negative” target window 610 (also referred to as “window 2”)has a frequency width h that is centered at a target LO-signal frequencyoffset of −F_(targ) from baseband. In an example, positive targetLO-signal frequency offset +F_(targ)=+500 MHz, negative target LO-signalfrequency offset −F_(targ)=−500 MHz, and frequency width h=200 MHz.Target LO-signal frequency offset F_(targ) may be set based on a baudrate of the transmit data (e.g., transmit data 110). In an example,F_(targ)=baud rate/64.

According to the receiver implemented embodiment, assuming opticaltransmitter TX2 transmits to optical receiver RX1, optical receiver RX1detects (after mixing) an LO-signal frequency offsetF_(tx2rx1)=F_(tx2)−F_(rx1) as indicated on plot 600, where F_(rx1)denotes the frequency of optical signal LO1 and F_(tx2) denotes thefrequency of optical signal LO2. Similarly, assuming optical transmitterTX1 transmits to optical receiver RX2, optical receiver RX2 detects(after mixing) an LO-signal frequency offset F_(tx1rx2)=F_(tx1)−F_(rx2)indicated on plot 600, where F_(tx1) denotes the frequency of opticalsignal LO1 and F_(rx2) denotes the frequency of optical signal LO2.Frequency offsets F_(tx2rx1) and F_(tx1rx2) are equal and opposite,i.e., F_(tx2rx1)=−F_(tx1rx2). If LO-signal frequency offsets F_(tx2rx1)and F_(tx1rx2) fall within respective target windows 610 and 605 asshown in FIG. 6, (i) optical receiver RX1 does not need to tune thefrequency of its optical signal LO1 to further adjust LO-signalfrequency offset F_(tx2rx1), and (ii) optical receiver RX2 does not needto tune the frequency of its optical signal LO2 to further adjustLO-signal frequency offset F_(tx1rx2). On the other hand, if LO-signalfrequency offsets F_(tx2rx1) and F_(tx1rx2) do not fall withinrespective target windows 610 and 605, optical receivers RX1 and RX2tune the frequencies of their respective optical signals LO1 and LO2 soas to drive their respective LO-signal frequency offsets F_(tx2rx1) andF_(tx1rx2) toward the nearest window.

In accordance with the loop-back embodiment, frequency plot 600 alsoincludes a dead-band window 615 for the LO-signal frequency offset,where the dead-band window is positioned between target windows 605 and610. In an example, dead-band window 615 is centered at a smallLO-signal frequency offset of +8 MHz, and has a width of 4 MHz, to matchthe small frequency offset imposed by frequency rotator 410 in a givenoptical transmitter. Assuming the loop-back arrangement in FIG. 1, theLO-signal frequency offset experienced in optical receiver RX1 is onlythe small frequency offset imposed by frequency rotator 410, which issufficient to spread average energy across photo-detectors PD1 and PD2.In accordance with the loop-back embodiment, if looped-back opticalreceiver RX1 detects an LO-signal frequency offset that falls withindead-band window 615, the optical receiver RX1 need not adjust thefrequency of optical signal LO1.

Consider an embodiment that combines the receiver implemented embodiment(target windows 605 and 610) and the loop-back embodiment (dead-bandwindow 615). In the combined embodiment, if the optical receiver detectsan LO-signal frequency offset that falls within any of windows 605-615,then the optical receiver need not adjust/tune the frequency of its(local) optical signal.

With reference to FIG. 7, there is a flowchart of an example method 700of controlling the LO-signal frequency offset between the frequencies ofoptical signals LO1 and LO2 performed in optical receiver RX1. Method700 assumes predetermined target windows 605 and 610, and dead-bandwindow 615, have been established and information defining the windowsis accessible to processor 514, e.g., is stored in memory 518. Method700 also assumes optical receiver RX1 receives modulated optical signal140 from optical transmitter TX2, and processes the (received) modulatedoptical signal in the manner described above, such that the carrierrecovery loop in receiver digital processor 308 determines the LO-signalfrequency offset between optical signals LO1 and LO2, denoted “F” inFIG. 7, and reports determined LO-signal frequency offset F to processor514 via signal 520.

At 702, 704, and 706, processor 514 receives LO-signal frequency offsetF (i.e., the determined/measured LO-signal frequency offset). Morespecifically, in operations 702, 704, and 706, processor 514clears/resets a previous received LO-signal frequency offset, waits forn seconds (e.g., 0.3 seconds) to allow the frequency of optical signalLO1 to build to an average value, and then reads the value,respectively.

At 708, processor 514 determines whether determined LO-signal frequencyoffset F is outside of (i.e., not in any of) windows 605, 610 and 615.If F does not fall outside of all of windows 605-619 (i.e., F is in oneof the windows), operations 702, 704, and 706 are repeated. If yes (Ffalls outside of all of windows 605-610), flow continues to 710.

At 710, processor 514 determines whether LO-signal frequency offset F isnearer/closer to positive window 605 or negative window 610. Forexample, processor 614 determines whether F is greater than a smallpositive LO-signal frequency offset A, in which case positive window 605is closer than negative window 610.

If it is determined that positive window 605 is closer, at 715 processor514 computes a frequency difference or error E between determinedLO-signal frequency offset F and the center frequency of the closesttarget window 605, i.e., E=F_(targ)−F. Flow proceeds to 725.

If it is determined that negative window 610 is closer, at 720 processor514 computes the frequency difference/error E between determinedLO-signal frequency offset F and the center frequency of the closesttarget window 610, i.e., E=−F_(targ)−F. Flow proceeds to 725.

At 725, processor 514 computes a small frequency tune amount e by whichthe frequency of optical signal LO1 should be tuned from its presentfrequency based on the frequency difference E, where e<E. Any suitablemethod to compute tune amount e may be used. In an example, tune amounte is chosen as half of the error E to reduce the possibilities of tuningoscillations. Thus, tune amount e is computed according to e=E/d, whered=2. Other values for d may be used.

At 730, processor 514 derives a new frequency F_(tune), for opticalsignal LO1 based on a previous frequency of the optical signal LO1 andtune amount e. In an example, F_(tune)=F_(tune) (previous)−e.

At 735, processor 514 limits the frequency tuning so that the frequencytuning does not drive the frequency of optical signal LO1 past either oftarget LO-signal frequency offsets+/−F_(targ). Thus: ifF_(tune)>F_(targ), then F_(tune)=F_(targ); or if F_(tune)<−F_(targ),then F_(tune)=−F_(targ). Operation 735 is optional, as LO-signalfrequency offsets>F_(targ) are acceptable. Processor 514 derivesfrequency control signal TC1 to tune the frequency of optical signal LO1to F_(tune), as determined at operations 730 and 735.

At 740, processor 514 waits p seconds (e.g. 5 seconds) for the tunedfrequency of optical signal LO1 to settle to the new tuned value, andflow returns to 702.

In the ensuing description, the term “signal” may be referred to as“frequency.” Thus, optical signal LO1, optical signal LO2, and modulatedoptical signal 112/140, may be referred to as optical frequency LO1,optical frequency LO2, and modulated optical frequency 112/140,respectively. Moreover, when referenced in connection with opticalreceivers RX1 and RX2, optical signal (or frequency) LO1 and opticalsignal (or frequency) LO2 may also be referred to more specifically as“optical LO signal (or frequency) LO1” and optical LO signal (orfrequency) LO2 because those optical signals (or frequencies) are usedas LO signals (or frequencies) in the optical receivers.

With reference to FIG. 8, there is a flowchart of an example transmitmethod 800 of generating modulated optical frequency 112 performed inoptical transmitter TX1.

At 805, transmitter digital processor 304 digitally modulates basebandtransmit data 110 to produce a baseband, i.e., stationary, digitalmodulation constellation. In an example, to digitally modulate thebaseband transmit data, digital processor 304 maps the baseband transmitdata to modulation symbols representative of the digital modulationconstellation.

At 810, transmitter digital processor 304 adds a small digital frequencyoffset to the digital modulation constellation to cause it to rotate ata rate equal to the added frequency offset. This generates opticalmodulation signals TIQ representative of the rotating digital modulationconstellation.

At 815, E/O converter 302 modulates optical frequency LO1 with opticalmodulation signal TIQ, to produce optical modulated frequency 112.Optical transmitter TX1 transmits modulated optical frequency 112.

In the loop-back arrangement, optical transmitter TX1 transmitsmodulated optical frequency 112 to optical receiver RX1, which receivesand processes the modulated optical frequency in accordance with method700. In method 700, because the same optical frequency, LO1, is used inoptical transmitter TX1 (as an optical carrier) and optical receiver RX1(as an LO signal), optical receiver RX1 will detect only the smallLO-signal frequency offset imposed by optical transmitter TX1. Thatsmall LO-signal frequency offset falls into dead-band window 615 (whichspans expected frequency offsets imparted by frequency rotator 410) and,therefore, optical receiver RX1 will not tune, i.e., change, thefrequency of optical LO frequency LO1.

With reference to FIG. 9, there is a flowchart of an example receivemethod 900 of controlling an LO-signal frequency offset between opticalLO frequency LO1 (used as an LO signal in optical receiver RX1) andoptical frequency LO2 (modulated by baseband data in optical transmitterTX2) performed in optical receiver RX1.

At 905, O/E converter 306 receives modulated optical frequency 140 fromoptical transmitter TX2. Modulated optical frequency 140 isrepresentative of optical frequency LO2 that was modulated to generatemodulated optical frequency 112. In other words, optical frequency LO2is an optical carrier for modulation.

Next operations 910-920 collectively detect/determine the LO-signalfrequency offset between modulated optical frequency 140 and optical LOfrequency LO1. This may be an average LO-signal frequency offset thatcorresponds to a difference between an average center frequency ofmodulated optical frequency 140 (and thus the average center frequencyof optical frequency LO2) and an average center frequency of optical LOfrequency LO1.

At 910, O/E converter 306 frequency mixes modulated optical frequency140 with optical LO frequency LO1 to frequency down-convert themodulated optical frequency toward baseband, to produce frequencydown-converted mixer signal products.

At 915, O/E converter 306 detects the mixer signal products to produceat or near baseband I and Q signals. The detected I and Q signalsrepresent a digital modulation constellation rotating at a rate equal toan LO-signal frequency offset between optical LO frequency LO1 andmodulated optical frequency 140 (and more specifically, the LO-signalfrequency offset between optical LO frequency LO1 and optical frequencyLO2).

At 920, receiver digital processor 308 determines the LO-signalfrequency offset based on the detected I and Q signals.

At 925, receiver digital processor 308 determines whether the LO-signalfrequency offset is in one of multiple predefined non-overlapping targetwindows 605 or 610 (or 615 in another embodiment) that cover respectivenon-zero LO-signal frequency offsets.

At 930, if the LO-signal frequency offset is determined not to be in oneof the target windows, receiver digital processor 308 tunes optical LOfrequency LO1 to drive the LO-signal frequency offset toward one of thetarget windows to ensure the LO-signal frequency offset is non-zero.Otherwise, if the optical LO-signal frequency offset is determined to bein one of the target windows, digital processor 308 does not adjust/tunethe optical LO frequency, i.e., the processor maintains the currentoptical LO frequency LO1 so that the LO-signal frequency offset remainswithin the one of the target windows.

Optical receiver RX1 repeats method 900 over time.

Features of the different embodiments described above, including thereceiver implemented embodiment and the loop-back embodiment, may beimplemented independent of one another or may be combined. For example,methods 700 and 900 may use only two target windows 605 and 610 or,alternatively, all three windows 605-615. Also, a given opticaltransmitter in communication with a given optical receiver may implementthe digital rotation described above, or may not implement the digitalrotation. Other combinations of features are possible.

Embodiments presented herein provide a simple method for obtaining goodperformance with digital modulation techniques, such as DP-BPSK or othertransmission formats where near zero LO-signal frequency offsets areproblematic. Removes performance impact of near zero LO-signal frequencyoffsets. The embodiments are straight-forward to implement andcompatible with low-cost systems where a single laser is used for boththe optical transmitter and the optical receiver. The embodiments may beconveniently retrofitted into existing systems that use conventionalDP-BPSK modulation formats and Mach-Zehnder control loops forQPSK/16-QAM.

In summary, in one form, a system is provided comprising: a laserconfigured to generate a tunable optical frequency; an opticaltransmitter configured to: map baseband data to symbols represented in adigital modulation constellation; add a frequency offset to the digitalmodulation constellation to cause the digital modulation constellationto rotate at a rate equal to the added frequency offset; and modulatethe optical frequency with the rotating digital modulation constellationand transmit the resulting modulated optical frequency; and an opticalreceiver configured to: receive the transmitted modulated opticalfrequency; and using the tunable optical frequency, detect the rotatingdigital modulation constellation conveyed by the received modulatedoptical frequency.

In another form, a method is provided comprising: generating a tunableoptical frequency; at an optical transmitter: mapping baseband data tosymbols represented in a digital modulation constellation; adding afrequency offset to the digital modulation constellation to cause thedigital modulation constellation to rotate at a rate equal to the addedfrequency offset; and modulating the optical frequency with the rotatingdigital modulation constellation and transmit the resulting modulatedoptical frequency; and at an optical receiver: receiving the transmittedmodulated optical frequency; and using the tunable optical frequency,detecting the rotating digital modulation constellation conveyed by thereceived modulated optical frequency.

In yet another form, a system is provided comprising: a laser configuredto generate a tunable optical frequency; an optical transmitterincluding: a symbol mapper to map baseband data to symbols representedin a digital modulation constellation; a frequency rotator to add afrequency offset to the digital modulation constellation to cause thedigital modulation constellation to rotate at a rate equal to the addedfrequency offset; and an electrical/optical (E/O) converter includingoptical modulator devices to modulate the optical frequency with therotating digital modulation constellation and transmit the resultingmodulated optical frequency; and an optical receiver configured to:receive the transmitted modulated optical frequency; and using thetunable optical frequency, detect the rotating digital modulationconstellation conveyed by the received modulated optical frequency.

The above description is intended by way of example only. Variousmodifications and structural changes may be made therein withoutdeparting from the scope of the concepts described herein and within thescope and range of equivalents of the claims.

What is claimed is:
 1. A system comprising: a laser configured togenerate a tunable optical frequency; an optical transmitter configuredto: map baseband data to symbols represented in a digital modulationconstellation; add a frequency offset to the digital modulationconstellation to cause the digital modulation constellation to rotate ata rate equal to the added frequency offset; and modulate the opticalfrequency with the rotating digital modulation constellation andtransmit the resulting modulated optical frequency; and an opticalreceiver configured to: receive the transmitted modulated opticalfrequency; and using the tunable optical frequency, detect the rotatingdigital modulation constellation conveyed by the received modulatedoptical frequency.
 2. The system of claim 1, wherein the opticalreceiver is configured to detect by: frequency down-converting thereceived modulated optical frequency toward baseband using the tunableoptical frequency; and detecting in-phase (I) and quadrature (Q) signalsrepresentative of the rotating digital modulation constellation from thefrequency down-converted received modulated optical signal.
 3. Thesystem of claim 2, wherein the optical receiver is further configured toremove the frequency offset from the rotating modulation constellationbased on the detected I and Q signals, to produce a stationary digitalmodulation constellation, and recover the baseband data from thestationary modulation constellation.
 4. The system of claim 1, whereinthe optical receiver is further configured to: detect the frequencyoffset based on the rotating digital modulation constellation; determinewhether the frequency offset is in a predefined dead-band window thatcovers a range of non-zero frequency offsets; and if the frequencyoffset is determined to be in the dead-band window, maintain the tunableoptical frequency so that the frequency offset remains in the dead-bandwindow.
 5. The system of claim 1, wherein the modulation constellationis a dual-polarization binary phase shift keying (DP-BPSK) modulationconstellation.
 6. The system of claim 1, wherein the optical receiver isfurther configured to: detect the frequency offset based on the rotatingdigital modulation constellation; determine whether the frequency offsetis in one of multiple predefined non-overlapping target windows thatcover respective non-zero frequency offsets; and if the frequency offsetis determined not to be in one of the target windows, tune the tunableoptical frequency to drive the frequency offset toward one of the targetwindows to ensure the frequency offset is non-zero.
 7. The system ofclaim 6, wherein the target windows include a positive target windowcentered at a positive target frequency offset from baseband and anegative target window centered at a negative target frequency offsetfrom baseband.
 8. The system of claim 7, wherein the target windowsfurther include a dead-band window centered at a dead-band targetfrequency offset between baseband and the positive target window.
 9. Thesystem of claim 6, wherein the optical receiver is further configured todetermine which of the target windows is nearest the frequency offset,wherein the optical receiver is configured to tune by tuning the tunableoptical frequency to drive the frequency offset toward the target windowdetermined to be nearest the frequency offset.
 10. The system of claim9, wherein the optical receiver is further configured to tune by tuningthe tunable optical frequency to drive the frequency offset toward thedetermined nearest target window such that the frequency offsetresulting from the tuning does not exceed the target frequency offsetfrom baseband at which the determined nearest target window is centered.11. A method comprising: generating a tunable optical frequency; at anoptical transmitter: mapping baseband data to symbols represented in adigital modulation constellation; adding a frequency offset to thedigital modulation constellation to cause the digital modulationconstellation to rotate at a rate equal to the added frequency offset;and modulating the optical frequency with the rotating digitalmodulation constellation and transmit the resulting modulated opticalfrequency; and at an optical receiver: receiving the transmittedmodulated optical frequency; and using the tunable optical frequency,detecting the rotating digital modulation constellation conveyed by thereceived modulated optical frequency.
 12. The method of claim 11,wherein the detecting at the optical receiver includes: frequencydown-converting the received modulated optical frequency toward basebandusing the tunable optical frequency; and detecting in-phase (I) andquadrature (Q) signals representative of the rotating digital modulationconstellation from the frequency down-converted modulated opticalsignal.
 13. The method of claim 12, further comprising, at the opticalreceiver, removing the frequency offset from the rotating modulationconstellation based on the detected I and Q signals, to produce astationary digital modulation constellation, and recovering the basebanddata from the stationary modulation constellation.
 14. The method ofclaim 11, further comprising, at the optical receiver: detecting thefrequency offset based on the rotating digital modulation constellation;determining whether the frequency offset is in a predefined dead-bandwindow that covers a range of non-zero frequency offsets; and if thefrequency offset is determined to be in the dead-band window,maintaining the tunable optical frequency so that the frequency offsetremains in the dead-band window.
 15. The method of claim 11, wherein themodulation constellation is a dual-polarization binary phase shiftkeying (DP-BPSK) modulation constellation.
 16. The method of claim 11,further comprising at the optical receiver: detecting the frequencyoffset based on the rotating digital modulation constellation;determining whether the frequency offset is in one of multiplepredefined non-overlapping target windows that cover respective non-zerofrequency offsets; and if the frequency offset is determined not to bein one of the target windows, tuning the tunable optical frequency todrive the frequency offset toward one of the target windows to ensurethe frequency offset is non-zero.
 17. The method of claim 16, whereinthe target windows include a positive target window centered at apositive target frequency offset from baseband and a negative targetwindow centered at a negative target frequency offset from baseband. 18.A system comprising: a laser configured to generate a tunable opticalfrequency; an optical transmitter including: a symbol mapper to mapbaseband data to symbols represented in a digital modulationconstellation; a frequency rotator to add a frequency offset to thedigital modulation constellation to cause the digital modulationconstellation to rotate at a rate equal to the added frequency offset;and an electrical/optical (E/O) converter including optical modulatordevices to modulate the optical frequency with the rotating digitalmodulation constellation and transmit the resulting modulated opticalfrequency; and an optical receiver configured to: receive thetransmitted modulated optical frequency; and using the tunable opticalfrequency, detect the rotating digital modulation constellation conveyedby the received modulated optical frequency.
 19. The system of claim 18,wherein the optical receiver includes: an optical/electrical (O/E)converter to frequency down-convert the received modulated opticalfrequency toward baseband using the tunable optical frequency anddigitize the frequency down-converted signal to produce a digitizedfrequency down-converted signal; and a digital processor to detectin-phase (I) and quadrature (Q) signals representative of the rotatingdigital modulation constellation from the digitized frequencydown-converted signal.
 20. The system of claim 19, wherein the digitalprocessor of the optical receiver is configured to: detect the frequencyoffset based on the rotating digital modulation constellation; determinewhether the frequency offset is in a predefined dead-band window thatcovers a range of non-zero frequency offsets; and if the frequencyoffset is determined to be in the dead-band window, maintain the tunableoptical frequency so that the frequency offset remains in the dead-bandwindow.